Hydrostatic slideway systems are commonly used as a work-table or a tool-table of metal-cutting machine tools. A hydrostatic slideway system includes a guideway unit having a pair of guide rails formed on an upper surface of a machine bed, and a slide-table unit having plural oil pockets formed respectively on a lower surface and a side wall of the slide-table unit. Hydrostatic bearing is well known for its inherently frictionless movement, owing to its capability of maintaining a fixed fluid film gap by supplying externally pressurized fluid into the bearing surfaces and avoiding the direct surfaces contact. Generally, a typical hydrostatic slideway system is composed of hydrostatic bearings, an external oil supply system and flow restrictors.
The pocket configuration can have great influence upon the performance of a hydrostatic bearing in various aspects, such as the flow rate, the maximum height limit, supporting capability, damping behavior and stiffness. There are several common types of pocket configuration used in the hydrostatic bearing, which are shallow pocket type, deep pocket type, groove type and island type, etc.
The external oil supply system, being an essential component of hydrostatic slideway system, is used to provide steady, temperature-stable and clean pressurized oil to the hydrostatic bearings.
Flow restrictors of hydrostatic bearings can be divided into two categories. One of which is the fixed restrictor, such as the capillary restrictors, the orifice restrictors, and the other is the variable restrictor, such as flow valve, pressure-sensing valve and membrane (or diaphragm) restrictor. As the fixed restrictors are easy to manufacture, they are vastly used in the hydrostatic slide way system. Flow valves, on the other hand, have been claimed to lead to considerable enhancement of the stiffness, nearly infinite value in a close neighbourhood of the working point. However, besides being costly, flow valves can be problematic in many ways. Without the use of restrictor, the corresponding hydrostatic bearings in the hydrostatic slide system will not be able to provide any stiffness and supporting ability. That is, by configuring the hydrostatic bearings in a hydrostatic slide system with matching restrictors, the maximal possible stiffness of the hydrostatic bearings can be provided.
Currently, the studies relating to the hydrostatic planar bearing are mostly focusing on the improvement of its pocket configuration and also on its restrictor configuration as well for the purpose of improving the performance of the hydrostatic planar bearing with regard to its load carrying capacity, maximum allowable speed limit, damping property and stiffness.
There exists some shortcomings of hydrostatic planar bearing in current hydrostatic slideway system:    (1) As the hydrostatic slideway system is designed with specific bearing pocket configuration and flow restrictor type. Its stiffness is of fixed value and is independent to external loading. While designing the hydrostatic bearing with the other pocket configuration and restrictor type, the bearing performance characteristics must be re-calculated theoretically.    (2) In the oil supply circuit of the traditional hydrostatic bearings, the restrictors are placed upstream of the feed hole of the bearing. For one thing, those restrictors cannot be installed sufficiently close to the bearing pocket's inlet, which results in time delay of their reaction to bearing film changes. Thus, ideally, the compensation ought to find place in the bearing gap itself.
There are already some studies for improving the aforesaid shortcomings. One of which is an aerostatic linear bearing with compensating device for aerostatic slide system disclosed in TW Pat. Pub. No. 435628. In this patent, a linear bearing is so configured that its use seven adjusting screws to define three surfaces of the same, that is, three for a vertical surface of the linear bearing and two for one side surface while another two for another side surface of the linear bearing. By adjusting the seven adjusting screws manually, the gap between the bearing race and the guide rail can be adjusted for compensating the load variation to the aerostatic linear bearing. Although the aforesaid patent is feasible, it is unsatisfactory in both accuracy and reliability.
Another study is disclosed in TW. Pat. Pub. No. 225576 and U.S. Pat. No. 5,104,237, entitled “Self-compensating hydrostatic linear motion bearing”. In this study, there are a pair of hydrostatic bearings arranged opposite to each other at the two sides of a guide rail that are connected respectively to an oil pouch by their corresponding oil channels. As a load is applied, the oil film clearances between the bearing carriage surfaces facing the guide rail will change according, and then the pressure difference between the oil films in the two gaps is compensated by the use of the oil pouch since it is able to communicate with the two bearings through the oil channels at the same time. Thus, self-compensation is provided. However, as oil pouch might respond quite slowly for compensating the pressure difference, the aforesaid design is not suitable for miniature slide systems.
Moreover, there is an integrated self-compensating hydrostatic bearing disclosed in U.S. Pat. No. 5,533,814, entitled “Low profile self-compensating hydrostatic thrust bearing”, U.S. Pat. No. 5,700,092, entitled “Integrated shaft self-compensating hydrostatic bearing”, U.S. Pat. No. 3,934,948 entitled “Self-pressurized and self-compensating hydrostatic bearing”, U.S. Pat. No. 5,281,032, entitled “Self-compensating hydrostatic bearing for supporting shafts and spindles and like for rotary and translational motion and method thereof”, U.S. Pat. No. 5,484,208, entitled “Elastically supported self-compensating flow restrictors for optimizing hydrostatic bearing performance” and TW Pat. Pub. No. 304221. In the aforesaid patents, there is a pair of hydrostatic bearings arranged opposite to each other at the two sides of a guide rail while being connected to each other by a groove, by that fluid is able to flow freely between the two opposite-arranged hydrostatic bearings. As a load is applied, the oil film clearances between the bearing carriage surfaces facing the guide rail will change according, and then the pressure difference between the oil films in the two gaps is compensated through the fluid communication in the groove. Thus, self-compensation is provided. However, pressure difference capable being compensated by the aforesaid design is limited to a very small range.
In addition, there is a self-compensating hydrostatic bearing disclosed in U.S. Pat. No. 5,980,110, entitled “Manifold for self-compensating hydrostatic bearing with integral”, U.S. Pat. No. 6,012,845, entitled “self-compensating hydrostatic bearing with tape” and PCT Pat. No. WO 99/53207, entitled “self-compensating hydrostatic bearing”. In the aforesaid patents, there is a pair of hydrostatic bearings arranged opposite to each other at the two sides of a guide rail while being connected to each other by a groove configured with a valve, by that fluid is able to flow freely between the two opposite-arranged hydrostatic bearings through the groove under the control of the valve. As a load is applied, the oil film clearances between the bearing carriage surfaces facing the guide rail will change according, and then the pressure difference between the oil films in the two gaps is compensated through the fluid communication in the groove under the control of the valve. Thus, self-compensation is provided. However, the oil circuit system for the aforesaid self-compensating hydrostatic bearing can be very complex and thus costly.